LT3748E 데이터 시트보기 (PDF) - Linear Technology

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LT3748E

100V Isolated Flyback Controller

Linear Technology 

LT3748

Applications Information

Synchronous Secondary Applications

Using a synchronous secondary controller such as the

LT8309 with the LT3748 is an excellent method to boost

converter efficiency and minimize heat, especially for lower

output voltages and higher output currents. However,

there are some important details to understand when

designing a synchronous application. First, although the

LT8309 controls a synchronous MOSFET in place of the

standard output rectifier, when properly configured that

synchronous MOSFET must turn off before the end of

the secondary conduction time. This ensures that there

is no reverse current sending power back to the primary

side of the transformer and no cross conduction once the

LT3748 GATE pin goes high on the next switching cycle.

As a result, the forward voltage drop of the secondary

MOSFET body diode is reflected back to the primary side

and sampled by the LT3748. In order to guarantee an

accurate sample and to maintain excellent line and load

regulation, the RDRAIN resistor of the LT8309 must be

optimized to allow the body diode to conduct long enough

to provide an accurate reflected voltage. To ensure ac-

curate output regulation the secondary MOSFET should

turn off at least 180ns before the secondary current goes

to zero. Figure 13 illustrates the expected waveform at

the primary side drain node and the LT8309 GATE pin us-

ing the circuit from Figure 21 with sufficient body diode

conduction time marked.

Because the body diode is conducting at the sampling

point for the LT3748 when the secondary current goes

to zero, the temperature coefficient of this body diode

should be compensated using the TC pin using the same

procedures outlined when a normal rectifier is used on the

secondary. The silicon junction of the body diode has a

negative temperature coefficient comparable to a standard

or Schottky diode and standard values specified earlier in

the applications section should be a good starting point.

100

PRIMARY SIDE

80

DRAIN VOLTAGE

60

BODY DIODE

CONDUCTION

40

LT8309

20

VGATE

0

012345678

TIME (µs)

LT3748 F13

Figure 13. Waveforms at LT3748 Primary Side MOSFET

Drain and LT8309 GATE Pin During Operation Illustrating

Optimum Body Diode Conduction Time

20

For more information www.linear.com/LT3748

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